An Approach (Mine, in fact)
First, crack dialogue.
Why? There's this weird ouroboros effect between characterisation and dialogue. Dialogue establishes character BUT dialogue emerges FROM character. Weirdly, you can exploit this by swimming a little against the tide.
Take two character stubs that you have in mind. They really don't need to be all that developed, could be as little as:
Jake: Fussy, lazy, prim.
Tony: Sharp, fidgety, sarcastic.
Then put them in a room and make them wait for something. This is an exercise I like to call "Rosencrantz and the Dumb Godot in Bruges" after scripts in which a pair of characters famously fill in time whilst waiting for something (note that in three out of the four the characters are spies/hitmen).
Now your brain is marinated in that free association space where you can let the dialogue flow. Two characters is enough to just get the focus right and allow them freedom to express themselves a bit better than in a crowd.
Tony is clearly going to be the one most ill-at-ease with waiting around so we'll give him something to do:
TONY is looking at his shoes. (NOTE: Why is he looking at his shoes? Well, he's fidgety. So I could have decided to have him be prepared and to bring a small rubber ball with him or something, or lucky and have him find a set up for a small gallery of targets at which to pitch rocks. I decided to make him both unprepared and unlucky so he is reduced to examining his shoes for inspiration.)
JAKE is laid out with a newspaper over his face, as if napping. (NOTE: So JAKE is more prepared than TONY. He brought a newspaper. He can probably tell that TONY is uncomfortable with nothing to do but is using his newspaper as an eyeshield. This indicates that JAKE is selfish.)
TONY: I think my shoes need cleaning.
JAKE (Under the newspaper): So get them cleaned.
TONY: I will. Not now, obviously, but I will, as soon as we get back to the city.
JAKE (Under the newspaper): Good idea.
TONY: I'd do it myself. I'd do it now. But, one, I don't think I do as good a job as a professional shoe shiner and two, I don't have any polish. I mean, who'd bring polish along on a job like this.
JAKE (Under the newspaper): Only a lunatic, for sure.
TONY: That's a terrible job... shoe shiner. Of course, there hasn't been the type of war that puts shoe shiners on the streets in a while. It's hard to find someone who'll shine your shoes. I think people regard the job as menial and humiliating. It's a shame. Most shoes just go around scuffed and dirty these days.
JAKE: Shoes are a lot cheaper than they used to be.
TONY: I know, right? I'm not sure that's a good thing. Shoes are important. They cushion you as you walk. Walking is vital to communication. If we couldn't walk then we couldn't have got here.
JAKE: We could have teleconferenced.
TONY: Mister Black didn't want to teleconference, he wanted face to face. He's old school, Mister Black.
And just pootle on like that for as long as you can. There are even dialogue writing exercises that can help you out with that.
Next, mix it up a bit.
Repeat the first process for a number of natural character pairs. The character pair is the basic atom of show-don't-tell character development and revelation.
Your next stage is to mix up the characters you have paired and put them in new situations. You might want to generate or acquire a number of situations where the two characters could be forced to wait. Also change up the pre-existent relationships between them. Make a male and a female character husband and wife (even if you don't intend that they end up that way in your proposed novel), have two characters who know each other well talk as if they have never met. Note that there is, in any two-handed conversation, an actor (the person who proposes the topic and trajectory of the conversation) and a reactor (the person who fills in with their own reactions to the actors actions).
After that you want to start widening the circle. The number of people involved in a scene up to about eight largely dictates what will happen between them due to the rules governing permutations thus:
- A person alone: with a completely dedicated and individual agenda. By default an unreliable narrator.
- Actor and Reactor: feeding off each other, pressing one another on, seeking companionship for its own end. A partnership is, by default, accepts a level of intimate intensity. Most prosaically this manifests as romantic love, but could equally well be obsessive hatred, or any other kind of odd thing in between. Each partner in a duo will tend to reveal more of themselves to the other than they intend as neither of them can be distracted by anyone else.
- The couple and the lodger: Always two people will be united in purpose and the last will be identified as "other", "outsider" or "leader".
- The double date: People pair off and a love of symmetery makes two couples who police each other's status for some kind of equilibrium obsessively. Unlike a couple left to their own devices any point of conflict will be deflated or defeated by the other unit. Subtext begins to become a thing.
- The troublemaker: As this is an odd number the "spare" character is free to sow chaos among the other four people's stable set up. The "outsider" can prevent couple A from stabilising couple "B" or vice versa. The fifth party is always likely to be a source of contention.
- The dinner party: Three couples mean that each person in any de facto couple takes an opportunity to stand on their own, the number of permutations of one-to-one interaction here is large but finite. Although friction in certain actions may lead to much ado about very little this party is ultimately static in its bonds.
- The King or The Fool: Here the odd person, introduced into the dinner party scenario, has an opportunity to unite everyone (either in ordered placidity or in terrifying hatred of them), or to sew the seeds of ultimate chaos splitting the party into smaller subdivisions.
- The Gathering: A bigger dinner party. The larger the group from this point the more that each person present has to shout louder and be clearer to "control" or "own" the conversation. Now people will be very careful about things like social norms projecting an image of someone as "the type of person they want to be", idiosyncrasy in character will decrease, the wisdom and madness of groupthink will emerge, interaction becomes increasingly pageant like. There may be a "leader" but instead of emerging naturally through personality and charismatic dominance the leader is more likely to be, in some sense, elected and therefore take on the mantle of governmental power rather than personal charisma.
From that point on the behaviour of crowds and leaders just multiplies. Two leaders and a crowd make a war, three leaders and a crowd suggests a courtroom, four leaders and a crowd suggest some kind of cold war. Always, the more people are added the more difficult it becomes for people to be established as "leaders". The tendency of large mobs is always towards chaos.
Given that overview I would tend to practice with scenes involving up to five characters. After that you're entering the realm of "crowd scenes" which are easier tackled when you have a lot of context.
Finally, to the matter of plot.
Some people, Elmore Leonard famously, have a very woolly idea of what the plot actually is, they like to assemble patchworks of character scenes into a narrative and leave it at that. This could be your thing.
If not, you should work out how your characters, who you should know quite a bit about by now, will achieve the ends of your initial plot. Some "plot problems" may seem intractable but I have never yet found one that can't be thought about until it is tweaked out of existence.
Anyway, the first two parts of this answer will probably get you going in the right direction. After that it's all about sewing up the plot holes.